1. Field of the Invention
The present invention relates to wireless communication devices, such as mobile information terminals, having wireless transmission and reception functions and used in wireless communication systems, such as a cordless telephone system, a PHS (Personal Handy-phone System), a WLAN (Wireless Local Area Network) and so on.
2. Description of the Related Art
Wireless communication systems such as a wireless telephone system, in which voice signals and other data are communicated between a base unit and a handset, are conventionally known. It has been proposed to constitute a front-end module for a wireless communication device used in such a wireless communication system, such that the front-end module includes a transmission circuit connected to a power amplifier (PA), a reception circuit connected to a low noise amplifier (LNA), and an antenna switch module (ASM) that switches the connection between an antenna and the transmission/reception circuits in a time divisional manner (see JP2002-290257A, for example). In a wireless communication device equipped with such a front-end module, it is possible to perform transmission and reception substantially at the same time by operating the antenna switch module at a high speed.
However, the antenna switch module can consume several tens of percent of electric power when in operation, and therefore, attempts have been made to allow transmission and reception to be performed without use of the antenna switch module, thereby to reduce the power consumption and/or the number of component parts to lower the cost. For example, there have been disclosed a technology in which the output end of the power amplifier is disconnected from both the power source line and the ground during signal reception so as to put the output end of the power amplifier in a high impedance state (JP2007-028459A), a technology in which a first phase shift line is provided between the power amplifier and the antenna and a second phase shift line is provided between the antenna and the low noise amplifier such that the impedance of the power amplifier as seen from the antenna is put in a substantially open state when the power supply to the power amplifier is shut off, and the impedance of the low noise amplifier as seen from the antenna is put in a substantially open state when the power supply to the low noise amplifier is shut off (JP2004-343517A), and a technology in which a phase shift circuit formed of a low-pass filter is used to adjust the phase shift angle so as to shift the impedance of the amplifier when the amplifier is not in operation from a substantially short-circuit state to a substantially open state (JP2010-057204A).
However, in the structure disclosed in JP2007-028459A, though the antenna switch module is omitted, a switch for disconnecting the output end of the power amplifier from the power source line is included instead of the antenna switch module.
Further, in the structure disclosed in JP2007-028459A, the output end of the power amplifier is put in a high impedance state during signal reception. However, there is a transmission impedance conversion circuit provided between the antenna and the power amplifier, and therefore, the signal received by the antenna during signal reception can flow to the transmission impedance conversion circuit. Thus, the structure disclosed in JP2007-028459A merely puts the output end of the power amplifier in a high impedance state, and does not put the impedance of the transmission circuit (here, the transmission impedance conversion circuit) as seen from the antenna in a high impedance state.
Consequently, there are problems such as increase in the power consumption due to operation of the switch, increase in the number of component parts and/or the circuit scale due to the presence of the switch, and degradation in performance in signal reception.
It is described in JP2004-343517A that addition of phase shift lines can cause the power amplifier to shift to a high impedance state for a reception band when the power supply to the power amplifier is shut off, and cause the low noise amplifier to shift to a high impedance state for a transmission band when the power supply to the low noise amplifier is shut off. However, according to the technology disclosed in JP2004-343517A, such an effect can be achieved only when a specific prerequisite is met. Namely, it is required that the power amplifier and the low noise amplifier exhibit an impedance consisting of a substantially pure reactance component and a reflection coefficient larger than or equal to 0.8 for the reception and transmission bands, respectively, when the power supply there to is shut off. Thus, the technology disclosed in JP2004-343517A has a problem in view of versatility, in that the power amplifiers and low noise amplifiers to which the technology is applicable are limited.
Further, it is described in JP2010-057204A that a phase shift circuit may be used to adjust the phase shift angle of the impedance of the amplifier when the amplifier is not in operation so as to shift the impedance from a short-circuit state to an open state. However, similarly to the technology disclosed in JP2004-343517A, the technology disclosed in JP2010-057204A also has a problem in versatility since the power amplifiers and low noise amplifiers that can be brought into a high impedance state by simply adjusting the phase shift angle are limited.